One of the substantial efforts in the further development of modern storage technologies is the increase of the integration density, so that the reduction in the structure sizes of the memory cells on which the memory devices are based is very important. Further efforts consist in developing novel memory cells which can be switched at relatively low voltages. A plurality of microelectronic elements and in particular memory cells which have a size of a few nanometers has been described in recent years. A concept for designing such memory cells is to arrange, between two electrodes, an active layer which can reversibly change certain properties, such as, for example, ferromagnetic properties or electrical resistance, depending on the voltage. Depending on the applied voltage, the cell can be switched between two states, so that one state can be assigned, for example, to the information state “0” and the other state can be assigned to the information state “1”.
Various memory cells having an active layer have been described in the prior art.
Compared with the cells which have a ferroelectric material between two electrodes, the cell which has, between two electrodes, an active layer which can change the electrical resistance depending on the applied voltage has the advantage that it has a substantially higher signal ratio between the OFF and ON state and need not be rewritten after the read process, since the reading of the state is not destructive.
Bandyopdhyay et al.: Applied Physics Letters, Vol. 82, pages 1215–1217 entitled, “Large conductance switching memory effects in organic molecules for data-storage applications” describes an active layer arranged between two electrodes and consisting of rose Bengal (4,5,6,7-tetrachloro-2′, 4′,5′,7′-tetraiodofluorescein) with a polyallylamine hydrochloride polymer. The electrode consists of indium tin oxide on glass. The production of the active layer is, however, very inconvenient and requires treatment in an oven for several hours in vacuo. In addition, the active layer is limited to the indium tin oxide electrode.
A further memory cell comprising an active material which exhibits switchable behavior is described in Yang et al.: Applied Physics Letters, Vol. 80, 2002, pages 2997–2999 entitled, “Organic Electrical Bistable Devices and Rewritable Memory Cells”. The active material consists of 2-amino-4,5-imidazoledicarbonitrile (AIDCN). The memory cell according to this prior art consists of a plurality of layers which have the following composition: an aluminium alloy deposited on glass, an AIDCN layer arranged thereon, a metal layer, a further AIDCN layer and a cathode. For switchability, this system requires the five layers described above, which makes the production very complex. A further disadvantage of the cells according to this prior art is that the cells can be switched only with aluminium electrodes and that the active layer can be applied only by means of vacuum vapour deposition.
According to C. P. Jarrett et al. in “Field effect measurements in doped conjugated polymer films: Assessment of charge carrier mobilities,” in J. Appl. Phys. 77(12), 15 Jun. 1995, pages 6289–6294, the conductivity and the field effect mobility are measured using metal-insulator-semiconductor field effect transistors and acceptor density measurements are carried out using metal-insulator-semiconductor diodes. The measurements were carried out on thin polymer films of the organic semiconductor poly(beta′-dodecyloxy-alpha,alpha′,alpha′,alpha″-terthienyl), which were doped with different conductivities using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an oxidizing agent. It was found that the field effect mobility and the conductivity of these films increased superlinearly on doping, whereas the transistor amplification, i.e. the ON/OFF ratio, decreased.